This invention relates to a wear-resistant sintered ferrous alloy useful for machine parts subjected to rubbing friction and a method of producing the same.
A typical example of metal parts that are forced to make continuous rubbing contact with another metal part is the rocker arm of an internal combustion engine. Usually the body of the rocker arm is formed by casting or forging, but the tip part where the rocker arm makes rubbing contact with a cam must be afforded with higher wear resistance. Therefore, it is usual to harden the tip portion of the rocker arm by a surface treatment such as carbrizing, nitriding or chromium plating, or alternatively to form the tip part separately from the main part of the rocker arm by chilled casting or by a powder metallurgy method and attach the tip part to the rocker arm body by soldering or by insert-casting.
As the performance requirements to the recent internal combustion engines for automotive uses have become more and more severe, there is the tendency to force the rocker arms to make rubbing contact with the cams under increased pressures. Then there arises a problem that the supply of lubricating oil into the interface between the cam surface and the rocker arm tip becomes insufficient while the cam rotation rate is low and hence the sliding speed of the rocker arm relative to the cam surface is low as occurs during idling or very low speed operation of the engine. The insufficiency in lubrication often results in serious wear or scuffing of the rocker arm tip made of a usual material.
It is possible to use a sintered porous alloy that is impregnated with oil to acquire a self-lubricating property and high resistance to wear. In that case, however, the sintered alloy contains relatively large amounts of special and costly metals such as W and/or Mo in order to possess such a high hardness as is sufficient for use in parts subjected to severe rubbing or friction. If the contents of such costly metals are decreased to reduce the cost of the sintered alloy, then it becomes necessary to subject the sintered alloy parts to a hardening treatment such as a heat treatment or a certain surface treatment whereby the production of the sintered alloy parts becomes complicated with inevitable rise in the PG,4 production cost. As another problem, the use of rocker arm tips made of a known sintered alloy which is highly resistant to wear often results in serious abrasion of the cams in sliding contact with the rocker arm tips.